Data Rate is the number of times data can be transferred per unit of time. Typical specifications for high-end memory include DDR-400, DDR2-800, DDR3-1600, DDR4-3200, and soon DDR5-6400, where each of those number represents millions of transfers per second (MT/s).
The above graphic illustrates a clock cycle in volts and time, so that more voltage would make the wave higher (more amplitude), while higher frequency would make it narrower (less duration). For both single and double data rate memory, bits are flipped between 0 and 1 by altering the voltage and thus the amplitude of one cycle of the signal.
Anything that happens per second can be expressed in Hz, but advanced discussions separate clock rate in MHz from data rate in MT/s. Using the above graphic, single data rate memory modified only the upper half of the wave, but double data rate also modified the lower half. Thus, while single data rate systems had the same number of data cycles as clock cycles per second, DDR memory has two data cycles for every clock cycle. The interface that connects DDR4-3200 for example runs at 1600 MHz.
Because data cycles are still a type of cycle and Hz is cycles per second, DDR4-3200 is often called “3200 MHz” despite its 1600 MHz clock. That description is not wrong, its nomenclature is simply less specific than 3200 MT/s. This explains the confusion of users who attempt to identify their data rate with software and see the clock rate reported rather than data rate.